1. Field of the Invention
The present invention relates to a modulation apparatus and a modulation method for orthogonally modulating a digital baseband signal, such as is utilized in a digital mobile communication and other communication system.
2. Discussion of Background Information
In a digital mobile communication system such as portable telephone, as disclosed in Japanese Unexamined Patent Publication No. 6-21991, a modulator is utilized. The modulator divides a transmitted signal into an in-phase component and a quadrature phase component, .DELTA..SIGMA. modulates each component then orthogonal modulates each component.
In the following description, a conventional modulator is explained with respect to the drawings. FIG. 1 is a block diagram illustrating a configuration of a conventional modulator.
In FIG. 1, .DELTA..SIGMA. modulator 2 converts a modulated signal input from input terminal 1 into a binary signal and outputs it to a multiplying circuit 3. Multiplying circuit 3 generates an amplitude modulated signal by multiplying the binary signal output from .DELTA..SIGMA. modulator 2 by a carrier signal and outputs the multiplied signal to output terminal 4.
Next, the operations of a conventional modulator having the above configuration are explained.
First a modulated signal input from input terminal 1 is .DELTA..SIGMA. modulated at .DELTA..SIGMA. modulator 2, then a binary .DELTA..SIGMA. modulated signal is output. Then an amplitude modulated signal is generated by multiplying the .DELTA..SIGMA. modulated signal by a carrier signal, which is output to another apparatus, via output terminal 4. As described above, a conventional modulator modulates a transmission signal into a digital orthogonal signal.
However in the conventional modulator as described above, a multiplication process is necessary for an orthogonal modulation under a high sampling frequency that is not avoidable to process the .DELTA..SIGMA. modulation. Because this requires a high-rate multiplier, the conventional modulator has a problem in that the consumed electric power becomes high.